CHAPTER2 Methods and Principles of Biological Systematics iological systematics (or taxonomy) is the theory and practice of Bgrouping individuals into species, arranging those species into larg- er groups, and giving those groups names, thus producing a classification. Classifications are used to organize information about plants, and keys can be constructed to identify them. There are many ways in which one might construct a classification. For example, plants could be classified on the basis of their medicinal proper- ties (as they are in some systems of herbal medicine) or on the basis of their preferred habitat (as they may be in some ecological classifications). A phylogeny-based classification, such as that followed in this book, attempts to arrange organisms into groups on the basis of their evolution- ary relationships. There are two main steps in producing such a classifica- tion. The first is determining the phylogeny, or evolutionary history. The second is basing the classification on this history. These two steps can be, and often are, separated, such that every new theory of relationships does not lead automatically to a new classification. This chapter will outline how one goes about determining the history of a group, and then will dis- cuss briefly how one might construct a classification, given that history. What Is a Phylogeny? As described in Chapter 1, evolution is not simply descent with modifica- tion, but also involves the process of separation of lineages. Imagine for a moment a population of organisms that all look similar to each other. By some process, the population divides into two populations, and these two populations go on to evolve independently. In other words, two lineages (ancestor–descendant sequences of populations) are established. We know this has happened because the members of the two new populations acquire, by the process of mutation, new characteristics in their genes, and possibly changes in their overall form, making the members of one popu- lation look more similar to each other than to members of the other popu- lation or to the ancestral population. These characteristics are the evidence for evolution. 9 10 CHAPTER TWO For example, a set of plants Year 10 will produce offspring that are Year 9 genetically related to their par- Year 8 ents, as indicated by the lines in Figure 2.1. The offspring will Year 7 produce more offspring, so that Year 6 we can view the population over Year 5 several generations, with genetic Year 4 connections indicated by lines. Year 3 If the population divides into Year 2 two separate populations, each will have its own set of genetic Year 1 connections, and eventually will Petals white, stems herba- Petals white, stems woody, Petals red, stems herba- acquire distinctive characteris- ceous, leaves non-hairy, leaves non-hairy, stamens five, ceous, leaves non-hairy, tics. For example, the population stamens five, fruit dry, fruit dry, seed coat smooth stamens five, fruit dry, on the right could develop red seed coat smooth seed coat smooth flowers, whereas the stems of the population on the left could Figure 2.1 The evolution of two hypothetical species. Each circle represents a plant. A mutation in the lineage on the left causes a change to woody stems, which is then become woody. Red flowers and transmitted to descendant plants.Woody-stemmed plants gradually replace all the woodiness are evidence that each herbaceous ones in the population. A similar mutation in the lineage on the right leads of the two populations consti- to a group with red petals. tutes a single lineage. The same process can repeat, and each of the new populations can divide (Figure 2.2). Again, we states. In this case, the character “flower color” has two know this has happened because of a new set of charac- states, white and red. The character “stem structure” also teristics acquired by the newly formed populations. has two states, woody and herbaceous, and so forth. All Some of the woody plants have fleshy fruits, and anoth- else being equal, plants with the same state are more er group has a spiny seed coat. Meanwhile, some of the likely to be related than those with different states. red-flowered plants now have only four stamens, and The critical point in this example, however, is that another set of red-flowered plants have hairy leaves. characteristics such as red petals and woody stems are The characteristics of plants, such as flower color or new, and they are derived relative to the ancestral popu- stem structure, are generally referred to as characters. lation. Only such new characters tell us that a new lin- Each character can have different values, or character eage has been established; retaining the old characteristic Year 18 Petals white, stems herbaceous, 17 leaves non-hairy, stamens five, fruit dry, seed coat smooth 16 15 Petals white, stems woody, 14 leaves non-hairy, stamens five, fruit dry, seed coat smooth 13 12 Petals white, stems woody, 11 leaves non-hairy, stamens five, fruit dry, seed coat spiny 10 9 Petals white, stems woody, leaves non-hairy, stamens five, 8 fruit fleshy, seed coat smooth 7 Petals red, stems herbaceous, 6 leaves non-hairy, stamens five, fruit dry, seed coat smooth 5 Petals red, stems herbaceous, 4 leaves hairy, stamens five, 3 fruit dry, seed coat smooth 2 Petals red, stems herbaceous, leaves non-hairy, stamens four, 1 fruit dry, seed coat smooth Figure 2.2 The same hypothetical set of plants as in Figure 2.1 after eight years and two more speciation events. METHODS AND PRINCIPLES OF BIOLOGICAL SYSTEMATICS 11 (white flowers, herbaceous stems, non-hairy leaves, five groups, one with herbaceous stems and red petals, the stamens, dry fruit, smooth seed coat) does not tell us other with woody stems and white petals. Each of these anything about what has happened. groups can also be divided into two groups. Thus the A character state that is derived at one point in time classification can be derived directly from the phylogeny. will become ancestral later. In Figure 2.2, woody stems Note that the hierarchy is not changed by the order in are derived relative to the original population, but are which the branch tips are drawn. The shape, or topolo- ancestral relative to the groups with fleshy fruits or gy, of the tree is determined only by the connections spiny seed coats. between the branches. We can tell the evolutionary A group composed of an ancestor and all of its “story” by starting at any point in the tree and working descendants is known as a monophyletic group (mono, up or down. This means that the terms “higher” and single; phylum, lineage). We can recognize it because of “lower” are not really meaningful, but simply reflect the shared derived characters of the group (synapo- how we have chosen to draw the evolutionary tree. morphies). These are character states that have arisen From this point of view, a plant systematics course could in the ancestor of the group and are present in all of its as well begin by covering the Asteraceae, which some members (albeit sometimes in modified form). This textbooks consider an “advanced” family, and then concept was first formalized by Hennig (1966) and working out to other members of the asterid clade, Wagner (1980). instead of starting with the so-called “primitive” fami- The diagrams of Figures 2.1 and 2.2 are cumbersome lies, such as Magnoliaceae and Nymphaeaceae. The lat- to draw, but can be summarized as a branching tree (Fig- ter simply share a set of characters thought to be ances- ure 2.3A). It is also inconvenient to repeat the ancestral tral, but these are combined with a large set of derived character states retained in every group, so systematists characters as well. commonly note only the characters that have changed, with tick marks placed on the appropriate branches to indicate the relative order in which the character states Determining Evolutionary History originated (Figure 2.3B). CHARACTERS, CHARACTER STATES, AND The shared derived characters in Figure 2.3B can be NETWORKS arranged in a hierarchy from more inclusive (e.g. stems woody or petals red) to less inclusive (e.g., leaves hairy, In the example in Figures 2.1, 2.2, and 2.3, we have seed coat spiny). These then lead to the obvious conclu- described evolution as though we were there watching it sion that the plants themselves can be arranged in a hier- happen. This is rarely possible, of course, and so part of archical classification that is a reflection of their evolu- the challenge of systematics is to determine what went tionary history. The plants could be divided into two on in the past. The relatives of an extant species must be (A) Petals red, stems herbaceous, Petals white, leaves non-hairy, Petals red, stems stems woody, stamens four, herbaceous, leaves non-hairy, fruit dry, seed Petals white, leaves hairy, stamens five, coat smooth stems woody, stamens five, fruit fleshy, leaves non-hairy, fruit dry, seed seed coat smooth stamens five, coat smooth fruit dry, seed Petals red, stems herbaceous, Petals white, coat spiny stems woody, leaves non-hairy, stamens five, leaves non-hairy, fruit dry, seed coat smooth stamens five, fruit dry, seed Petals white, stems herbaceous, coat smooth leaves non-hairy, stamens five, fruit dry, seed coat smooth (B) Fruit fleshy Seed coat spiny Stamens four Leaves hairy Stems woody Petals red Figure 2.3 (A) A simple way to redraw the pattern of change shown in Figure 2.2. Full descriptions are provided for each of Petals white, stems herbaceous, the ancestors and their descendants.